Electrochemical processes that form oxide films on such metals as aluminum, tantalum, niobium, etc. using liquid electrolyte are well known. See, for example, "Anodic Oxide Film", L. Young, Academic Press, London, New York, 1961.
Also, other anodization techniques have been also proposed. For example, Smyth suggests in "Solid-State Anodic Oxidation" (Journal of Electrochemistry, 113, 19 (1966) that anodization may be made to occur at the expense of local oxygen content of MnO.sub.2 in a Ta-Ta.sub.2 O.sub.5 -MnO.sub.2 solid electrolyte capacitor. Moreover, Miles and Smith suggest in "The Formation of Metal Oxide Films Using Gaseous and Solid Electrolytes" (Journal of Electrochemical Society) 110, 1240 (1966)) that anodization may be made to occur, among other ways, by transport of oxygen dissolved in metal deposited on an oxidized, anodizable conductor.
This invention differs from these earlier disclosures in that anodization occurs when external oxidant infuses through a permeable conductor to a solid interfacial region between electrodes.